Rijke type combustion arrangement and method

ABSTRACT

A process of producing heat energy for use in heat exchange with other fluids and substances so as to impart said heat energy to said fluid or substances which includes several steps. In a first step, a mixture of fuel and oxidant is ignited in a combustion zone or zones ( 16, 18, 98, 122 ) to create a combusted fuel mix which in the form of shockwaves are carried away from the combustion zone or zones ( 16, 18, 98, 122 ) to provide a low pressure area within the combustion zone or zones ( 16, 18, 98, 122 ). In a second step, said low pressure area causes partial return of the combusted fuel/oxidant mixture from a location remote of the combustion zones ( 16, 18, 98, 122 ) and also causes a new change of oxidant/fuel mixture to be transferred from the combustion zone or zones ( 16, 18, 98, 122 ) whereby incoming and returning hot gases are caused to ignite automatically without the aid of the ignition device used in the first step thereby causing a series of pulses or oscillations of successive return of hot gases and further changes of fuel/oxidant mixture to provide a process of production of energy which is self-sustaining and continuous.

FIELD OF THE INVENTION

This invention relates to the novel geometrical division of resonant combustors in Rijke type thermo acoustic combusting processes and, in particular, combustion-induced flow oscillating combustors. The invention is directed towards Rijke type resonant combustion processes with minimal NOx emission and minimal CO emissions. and increased heat transfer, preferably with application to heat generation-transfer.

BACKGROUND OF THE INVENTION

Much effort has gone Into research on improving energy efficiency and pollutant emissions from combustion processes. These have addressed the energy efficiency and the reduction of sulfur oxide gases (SOx), the elimination, of unburnt hydrocarbons and carbon monoxide (CO) and in particular and most importantly the oxides of nitrogen (NOx).

Oxides of nitrogen NOx is used to refer to NO and NO2. NO is the primary form in combustion products (typically 95 percent of total NOx). NO is subsequently oxidized to NO2 in the atmosphere. Nitrogen oxide formation occurs through three reaction paths, each having unique characteristics is responsible for the formation of NOx during combustion processes: NOx reduction is the area of most concern today. Thermally produced NOx is the largest contributor to these types of emissions. Thermal NOx is produced during the combustion process when nitrogen and oxygen are present at elevated temperatures. The two elements combine to form NO or NO2. NOx is generated by many combustion processes. It combines with other pollutants in the atmosphere and creates O3, a substance known as ground level ozone. Ground-level ozone is serious because it can aggravate asthma and cause lung inflammation and chest pains and has dangerous long-term effects, it is also a key ingredient of urban smog.

NOx emissions do not form in significant amounts until flame temperatures reach 1500° C. Once that threshold is passed, however, any further rise in temperature causes a rapid increase in the rate of NOx formation. NOx production is highest at fuel-to-air combustion ratios of 5-7% O2 (25-45% excess air). Lower excess air levels, in conventional combustion systems, starve the reaction for oxygen and higher excess air levels drive down the flame temperature, slowing the rates of reaction.

Conventional processes for reducing NOx emissions include pre-combustion or post combustion processes which are attained through fan forced steady combustion processes. In the current global warming climate this is undesirable.

The purpose of this invention is to limit or prevent the emissions of NOx into the atmosphere, and to increase the heat transfer, in the process of burning fuel to drive a process to produce a product by getting the released energy into the process as efficiently as possible. Most combustion engineers have concentrated on the combustion process itself. They have ignored the importance of the geometry of the combustion chamber, and as a consequence of what happens to the released energy.

There have been efforts, to improve the heat transfer from fluids by flow pulsation, e.g. Milburn Ind Eng Chem Fundam 2:62 (1970). A conventional mechanism to enhance the heat transfer has been obtained through solenoid switching of the flow direction or pulsing the flow via a piston or set of pistons. The increase in the heat transfer coefficients obtain in these tests was of the order of 70%. Similar work has been done on enhancing mass transfer coefficients. Chandhok et al. AlChE Journal 36 1259 (1990) showed that under the correct conditions the mass transfer coefficient could be increased by 2 orders of magnitude. The main influencing factor is not the frequency of the pulsations, but the amplitude, the higher the amplitude the larger the increase in the transfer coefficient. The amplitude is the magnitude of change in the oscillating variable, with each oscillation, within an oscillating system. For instance, sound waves are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation. If a graph of the system is drawn with the oscillating variable as the vertical axis and time as the horizontal axis then the amplitude may be measured as the vertical distance between points on the curve.

Pulsating and oscillating combustion is the consequence of a combustion instability that is driven into resonance by the geometry of the burner. The resonance is the tendency of a system to oscillate at maximum amplitude at certain frequencies, known as the system's resonance frequencies (or resonant frequencies). At these frequencies, even small periodic driving forces can produce, large amplitude vibrations because the system stores vibrational energy. When damping is small the resonance frequency is approximately equal to the natural frequency of the system, which is the frequency of free vibrations. Resonant phenomena occur with all type of vibrations or waves. Resonant systems can be used to generate vibrations of a specific frequency, or pick out specific frequencies from a complex vibration containing many frequencies. Normally, combustion engineers avoid combustion-generated oscillating instabilities at all costs, since they can very quickly lead to catastrophes if the combustion chamber is not designed to capture and transfer the energy of the oscillating combustion.

Rijke type combustion refers to use of a combustion tube having open ends and a heat source adjacent an inlet end which is heated gauze. The Rijke type does not require use of a valve unlike other conventional pulsating and oscillation combustion systems and provides acoustic mode excitation wherein air passing up the tube is heated and expanded producing pulses of air which starts this combustion arrangement or system into oscillation at is natural frequency.

It was always thought that a Rijke Tube in the absence of an electrical component had to be vertical and straight with the convection current created by the hot gases flowing upwards, circular and uniform in cross-section, in order to work.

SUMMARY OF THE INVENTION

The present invention provides an efficient and effective means of harnessing Rijke type technology in a novel and inventive fashion.

The invention provides in a first aspect a process of producing heat energy for use in heat exchange with other fluids and substances so as to impart said heat energy to said fluids or substances which includes the steps of:

-   -   (i) igniting a mixture of fuel and oxidant in a combustion zone         or zones caused by an ignition device located in said combustion         zone or zones to create a combusted fuel mix which in the form         of shockwaves are conveyed away from the combustion zone or         zones to provide a low pressure area within the combustion zone         or zones; and     -   (ii) said low pressure area causing partial return of the         combusted fuel/oxidant mixture from a location remote of the         combustion zone or zones and also causing a new charge of         oxidant/fuel mixture to be transferred into the combustion zone         or zones whereby incoming and returning hot gases are caused to         ignite automatically without the aid of the ignition device used         in step (i) thereby causing a series of pulses or oscillations         of successive return of hot gases and further charges of         fuel/oxidant mixture to provide a process of production of         energy which is self sustaining and continuous.

The term “combustion” as used herein not only includes ignition of the fuel/oxidant mixture with or without flames but also includes detonation and/or deflagration of the fuel oxidant mixture which may take place in a porous membrane adjacent the combustion zone as described hereinafter.

Preferably the combustion zone is separated into an inner and outer combustion zone wherein ignition devices are located in each of the inner and outer combustion zone(s). In another arrangement there may be provided a multiplicity of separate combustion zones which are in, communication with each other. In this case only a single ignition device may be required.

The automation ignition occurs at a detonation zone adjacent the Combustion zone(s) and the detonation zone may be formed by a porous membrane located in inlet(s) of a planar member having spaces which form part of the combustion zone(s).

In accordance with a second aspect of the invention there is provided a Rijke type combustion arrangement for use in the process described above having at least one combustion zone having an associated ignition device wherein the or each combustion zone has one or more inlets or entrances and a flame retainer in the form of a porous membrane located in or adjacent to the or each inlet or entrances and there is further provided one or more conveying zones located above the porous membrane for conveying hot gases away from the combustion chamber or space.

Preferably in a first embodiment there are provided inner and outer combustion zones each having an associated ignition device and a tube assembly having at least two tubes forming a plurality of separate conveying zones or passageways. Each tube may have an inner end wherein the inner ends of each of the tubes are each open to a different one of the inner and outer combustion zones and each porous membrane is in fluid communication with an associated inner end of each of the tubes. This arrangement is shown in FIGS. 1-7.

In yet another variation or second embodiment there may be provided a combustion′ arrangement which only has one conveying zone which is a single chamber located above the porous membrane and a combustion zone located closely adjacent to the porous membrane which may include the ignition device. This arrangement is shown in FIGS. 9-11.

Hereinafter the description will concentrate on the use of inner and outer combustion chambers as set out in the first embodiment described above.

Preferably each of the tubes is in flow communication with each other at an outlet thereof.

Preferably there is provided a planar member adjacent each of the inner and outer combustion chamber having apertures therein which form the inlet(s) of the inner and outer combustion zones which may be combustion chambers.

Preferably the apertures are each defined by a cylindrical wall and the flame retainers are located between opposed ends of the apertures.

The combustion arrangement may also include a frequency adjustment device at each outlet of said at least two tubes as well as adjacent a respective inlet of the inner and outer combustion zones.

More specifically the combustion arrangement may include frequency adjusting sleeves which are movably or slidably mounted relative to the apertures of the planar member thereby to allow adjustment of the inlet orifice depth which is the distance between a lip of the frequency adjusting sleeves and the flame retainers. Frequency adjusting sleeves may also be movably or slidably mounted to each tube adjacent an outlet thereof.

Preferably the tube assembly having the said at least two tubes has an even number of tubes, each tube having an internal chamber and wherein half of said internal chambers are open to the inner combustion chamber and the other half being open to the outer combustion chamber.

Preferably the inner combustion chamber and outer combustion chamber are separated by a partition which is continuous and more preferably has a peripheral edge which is serpentine.

In one embodiment there may be provided a ring of tube internal chambers which are alternately open to the outer combustion chamber and the inner combustion chamber.

In another arrangement there may be provided a plurality of tube internal chambers which form an outer set of tube internal chambers and an inner set of tube internal chambers which are concentric with the outer set. In particular the inner ends of the outer set of tube internal chambers are open to the outer combustion chamber and the inner ends of the inner set of tube internal chambers are open to the inner combustion chamber.

Preferably the combustion arrangement includes combustion ignition means. The combustion ignition means may comprise a spark creator such as a spark plug or a pilot flame which preferably are located in or adjacent the inner combustion chamber and the outer combustion chamber respectively.

Preferably the combustion arrangement includes a fuel plenum and the inlet(s) of the combustor are open to the fuel plenum.

Preferably the combustion arrangement includes a fuel mixing chamber which is open to the fuel plenum for feeding fuel to the fuel plenum.

Preferably the mixing chamber may have obstructions therein which causes fuel in the fuel mixing chamber to mix as it circulates in the fuel mixing chamber.

The combustion arrangement may further comprise urging means for forcing fuel and/or oxidant into the fuel mixing chamber. The urging means may comprise one or more suitably chosen (with respect to pressure delivery and flow rate) fans or similar arrangement.

The combustion arrangement may include an inlet acoustic decoupling and/or an outlet acoustic decoupling device.

In one embodiment of the combustion arrangement the tube assembly has fluid holding arrangements such as tubes or coils placed in at least one of the tubes through which a fluid to be heated may flow.

Preferably the chambers are generally circular in cross-section. Alternatively, the tube internal chambers may be square, rectangular, triangular or other shape in cross-section. The cross-sectional area of the internal chambers may be constant or, alternatively, may vary. The tube internal chambers may therefore be aligned non-vertically; they may be curved or angularly deviated. They may be mono-walled or cavity-walled; they may be non-circular and irregular in cross-section and may vary throughout their length. The tube internal chambers may be straight, bent, S-shaped, U-shaped or coiled or otherwise conformed so as to reduce or increase the distance over which the tube internal chamber extends away from the combustor. The tube internal chambers may communicate with each other.

The invention extends to a combustion arrangement further including a substance delivery means for delivering a gas, fluid or other substance to be heated to a heat exchanging surface of the combustion arrangement. The substance delivery means may comprise a fan, pump, a belt, a chute, a chamber or other arrangement suitable to maintain the substance in heating contact for a desired period of time, with the heat exchanger surface. In a preferred embodiment, the substance delivery means comprises a pump or auger-like arrangement to advance the substance in a desired path or direction. One or more combustor arrangements according to the present invention may be located inside a housing, the housing adapted to receive the substance and to circulate around or inside or outside of one or more tube internal chambers to thereby mix the substance and provide even heating throughout.

Use of Rijke type combustors that are non-vertical provide greater efficiency in heat transfer by decreasing the discharge of heat through the vertical exhaust. Larger areas may be suitably provided for heat transfer. Additionally, the use of modular arrangements allows for complex devices and configurations to be provided for a significant transfer of heat with time. Different shapes facilitate different methods and the use of non-circular Rijke type combustors may also be of considerable advantage. The invention actively utilises combustion instability to gain a number of advantages. The resonance driving of combustion causes the system to oscillate at maximum amplitude at certain frequencies, locks the combustion instability into a very stable repetitive pattern at the combustor arrangements resonant frequency, which can be anywhere between 15 Hz to 20,000 Hz, but more frequently lies in 400 Hz and 8,000 Hz range. When the acoustic pressure wave exceeds the pressure drop of the gases through the combustion chamber inlet(s), the burner becomes self-aspirating and there is no need for a fan to continuously supply the combustion mixture. The flame is not continuous but a series of discrete flamelets that are ignited on the hot remnant gases of prior flamelets. The invention therefore actively utilises the instability to gain a number of advantages.

As a result of the pressure fluctuations, the overall heat transfer and mass transfer coefficients may be two orders of magnitude higher than conventional systems. The implications of this are that the size of the equipment can be reduced, i.e. the heat transfer area can be more than halved to carry out the same duty as a forced convection conventional combustion system supplying heat to an industrial process.

The acoustic pressure wave causes the gases and material in the primary and the tube assembly to oscillate rapidly. This has at least three known effects that cause an increase in the transfer rates:

-   -   1. The boundary layer never gets a chance to establish itself         and, consequently it is always trying to develop;     -   2. The heat transfer surfaces experience micro-vibrations; and     -   3. The temperature and concentration gradients at right angles         to the means flow are periodically extremely large.

These effects are more than additive and, as a result, the transfer coefficients are often at least two orders of magnitude greater than in conventional systems. There has been experimental evidence of a fourth effect increasing the heat transfer rate. This has also been backed up by a theoretical investigation by Merkin & Pop Int. J of Heat and Mass Transfer 43 611-621 (2000). In this effect the thermal wave at right angles to the flow field extends way beyond the physical boundaries of the combustion chambers walls far into the material or substance being heated. The extent of this distance is more than an order greater than the boundary layer thickness in conventional heat transfer. An added advantage of the micro-vibrations on the combustor arrangement and heat transfer equipment walls is, that the heat transfer surfaces tend to be self-cleaning, (i.e. as new performance throughout the life of the plant). In many process operations fouling of equipment can represent an added cost as well as being a bottleneck in the process. The inventors, however, offer this by way of hypothesis and do not seek to be bound to any one or more explanations as to how the invention works.

The Rijke-type combustor arrangement detailed in this invention can be made to operate at any orientation. Many variations have been tried and proven by the inventor.

The combustion arrangement and combustion chamber(s) need not be round and can be rectangular provided the correct aspect ratio is chosen to match the harmonic frequencies generated by the length and geometry of the Rijke type combustor arrangement. This is achieved by ensuring that the major length of the combustors (L) is a whole number multiple of both the inlet orifice depth (α), the flame retainers thickness (t) and the fundamental harmonic oscillation frequency (f) whereas the inlet orifice depth is α={(½L*n/f)−(50*t)}/100 where n=the number of nodes. By these constructions, it has been found that stable oscillating combustion can be achieved, over a wide range of operating conditions, with there being a band or bands of oscillating frequencies at such operations can be achieved. With this particular geometric construction in accordance with the invention, it is possible to obtain a higher combustion frequency with increased heat transfer, higher than conventional designed equal volume combustors. In the case of the non vertical combustor chamber, a fan may be required to provide the flow normally provided by the natural convection or buoyancy in vertical sections of the combustor arrangement. In a preferred embodiment the thermo acoustic or Rijke type combustor arrangement enables heat exchange for hot water or steam production to be done in a more energy efficient manner and with limited NOx emission to the atmosphere. The small physical size of the equipment, compared with conventional equipment may allow retrofitting of existing burner systems with a more energy efficient and greener alternative.

In a preferred embodiment the thermo acoustic combustor arrangement enables heat exchange and or sterilisation or pasteurization of soil to be done in situ as a cultivation/sterilisation process in a single operation, which is expected to provide an environmentally friendly and cost-efficient alternative for the horticultural industry.

The process involves the steaming or heating of the topsoil or growing medium to a temperature of approximately 86° C. This is enough to cause extermination of the pathogenic pests but without heating the soil to levels that are considered harmful to the beneficial nitrogen fixing bacteria, necessary for normal plant growth.

A soil sterilisation/pasteurisation arrangement may be constructed that can be stationary, tractor mounted or self-propelled. It may process soil at the same time it is tilled.

Throughout the specification, the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features. Those of skilled in the art will therefore appreciate that, in light of the instant disclosure, various modifications and changes can be made in the particular embodiments exemplified without departing from the scope of the present invention. All such modifications and changes are intended to be included within the scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

To assist in understanding the invention and to enable a person skilled in the art to put the invention into practical effect, preferred embodiments of the invention will be described by way of example only with reference to the accompanying drawings, wherein.

FIG. 1 shows a perspective view of the Rijke type combustion arrangement in a preferred embodiment in accordance with the invention;

FIG. 2 shows an exploded perspective view of the Rijke type combustion arrangement of FIG. 1;

FIG. 3 shows a sectional view of the Rijke type combustion arrangement in accordance with the invention;

FIG. 4 shows a typical plan view of a divided combustion arrangement;

FIG. 5 shows a typical section of the combustion arrangement;

FIG. 6 is an exploded perspective view of the combustion arrangement, of the invention in a vertical orientation;

FIG. 7 is a longitudinal sectional view of the combustion arrangement shown in FIG. 6;

FIG. 8 is a perspective view of another embodiment of the Rijke type combustion arrangement of the invention with part of the external skin or casing removed for convenience;

FIG. 9 is an external perspective view of another embodiment Of the combustion arrangement shown in FIG. 8;

FIG. 10 is a sectional view through line A-A of FIG. 9; and

FIG. 11 is a perspective view of the combustion arrangement shown in FIGS. 9-10 with part of the external skin or casing removed for convenience.

The Rijke combustion arrangement and method described in the present application provides for complete combustion of a combustible gas mix and preventing or limiting the emissions of oxides of nitrogen NOx.

In FIG. 1 there is shown a Rijke type thermo acoustic combustion arrangement 10 having a cylindrical body 37 and fuel oxidant inlet end 50 and an exhaust end 54. At the inlet end 50 there is provided a support frame 49 having a pair of beams 47 and 48 for supporting body 37 in a vertical orientation wherein each beam 47 and 48 contacts the ground (not shown). There is also provided peripheral plate 46 welded to beams 47 and 48 and attached to body 37 by fasteners 45. There is also provided fixed conduit or tube 60 having inlet 53 for fuel and inlet 51 for oxidant which each have peripheral mounting flanges 44 for attachment to a source of fuel (not shown) and oxidant (not shown). Plate 46 is integral with or attached to cylindrical inlet body mixing tube 52.

As shown in FIG. 2 inlet body 52 has a central plate 28 for attachment to corresponding central plate 33 by fasteners 42 wherein fuel oxidant plenum 40 surrounds plate 28.

At the outlet end 54 of combustion arrangement 10 there is provided exhaust body or tube 41 and peripheral end plate 55 shown in FIGS. 2 and 6 attached to end plate 12 shown in FIG. 6 by fasteners 13 shown in FIG. 1 passing through aligned apertures 13A and 13B also shown in FIG. 6.

In FIG. 2 there is shown observation tubes or ports 62 and mounting plates 63 and 64 made of mesh for observation tubes 62. Exhaust tube 41 also has mounting flange 65 for attachment to plate 64 by fasteners 67. Also in FIG. 2 there is shown a fuel diffusing screen 58 of metallic mesh with small apertures, e.g. of 30-100 microns and more preferably 50-80 microns, to enable the equal distribution of fuel and oxidant to the plenum 40. The feed conduit or tube 60 may be in fluid communication with the fuel or oxidant source with the agency of a fan, pump or other device that may include a valve assembly and regulating system (not shown) which controls the amount of oxidant or fuel fed into feed conduit 60. Both plates 63 and 64 are also in the nature of screens formed from mesh as described above for screen 58.

Also as best shown in FIG. 6 there is also provided a central plate 17A having a set of inlet apertures 16A for cold water entering tubes 30 shown in FIG. 7 and a set of outlet apertures 17 for hot steam exiting through tubes 30.

Also as shown in FIG. 2 which shows the interior, of body 37 there is provided a combustor 12 and a tube assembly 14. The combustor 12 is divided into an outer combustion zone or chamber 16 and an inner combustion zone or chamber 18. The outer combustion chamber 16 and the inner combustion chamber 18 are divided by a continuous serpentine partition wall 20 having a serpentine shape as shown in FIG. 4. The outer combustion chamber 16 is surrounded by a peripheral wall 32 also shown in FIG. 4 which is attached to adjacent peripheral flange 31 of mixing tube 52 by fasteners 29. There is also provided central disc 33 which is concentric with the surrounding circular wall 32 and is located at the centre of the inner combustion chamber 18.

The tube assembly 14 comprises geometrically arranged tubes 22. The tubes 22 are arranged in a ring. The tubes 22 have inner ends 38 at their fuel/oxidant inlet ends shown in FIG. 5. The combustor 12 and the internal chambers of tube assembly 14 are in communication with each other as will be discussed in more detail herein-below.

The combustor 12 has planar member 26 with a number of geometrically arranged apertures 24 therein, which define fuel/oxidant inlets 66 to the combustor 12 as shown in FIG. 5. The apertures 24 in planar member 26 have opposed recesses 26A which determines the thickness of porous membranes 28. The wall 20 meanders between the apertures 24 so that adjacent apertures 24 are alternatively in the outer combustion chamber 16 and the inner combustion chamber 18.

Flame retainers in the form of porous membranes 28 are located in the apertures 24 as also shown in FIG. 5. The combustor 12 includes frequency adjusting sleeves 56 shown in FIG. 5 which are screw-threadingly fixed inside the apertures 24. The frequency adjusting sleeves 56 are displaceable towards and away from the porous membranes 28, thereby varying the distance from an inlet tip 48 of the sleeves 56 to the porous membrane 28. This distance is referred to as the inlet orifice depth (α).

The porous membrane 28 is of ceramic or metallic mesh. The membrane has, a preferred thickness (t) of 1-10 mm. The porous membrane 28 is provided in the flow path 42 of the fuel/oxidant mixture to be burnt shown in FIG. 3. The membrane 28 allows a controlled charge of the combustible fuel/oxidant mixture into the primary combustor 12 to be ignited whereby the membrane 28 provides resistance and partial closure, thereby preventing backflow under back pressure from the combusted fuel in the combustor 12 until, on reduction of back pressure due to exhaust of combusted gas, the pressure drop at the inlet 66 is sufficient to allow again fuel access to the combustor 12. This cycle is repetitively executed so that combustion occurs in a continuous rhythmic or pulsing fashion. The combustor 12 communicates with fuel/oxidants plenum 40 via the porous membrane 28.

The membrane 28 may be formed from thin filaments of sintered metal for example non woven FeCrAl alloy steel. They have very high porosity between 80-90% and have very high flow rates i.e. up to 20 times higher than the conventional media. The membrane 28 is very strong and can be used in temperatures up to 1300° C.

The inner ends 38 of the tubes 22 are each in register with a different aperture 24. The outlets 23 of the tubes 22 at their distal ends from inlets 38 are surrounded by frequency adjusting sleeves 30 as shown in FIGS. 2 and 3. The sleeves 36 are slidingly adjustable on the tubes 22. The sleeves 36 abut each other.

The fuel/oxidant plenum 40 is open to the fuel/oxidant mix from mixing chamber 52. The fuel/oxidant plenum 40 and the mixing chamber 62 is separated by the fuel diffusing screen 58. Geometrically formed blocks of material (not shown) are inserted into the fuel and oxidant mixing chamber 52 to disturb flow of fuel and oxidant in the chamber 52 to enhance uniform mixing of the fuel and oxidant in the chamber 52.

The walls 20, 32, 33 are oriented normal to the mean flow direction 42 of the fuel and oxidant shown in FIG. 3. It has been found that, as a consequence of the variation in width in the combustion chambers 16, 18 created by the serpentine like configuration of the serpentine wall 20 there is not a single well defined acoustic resonant frequency, but the combustor 12 is capable of resonant type operation over a wide range of frequencies. It is envisaged that extension of the range of acoustic resonant frequencies in an embodiment can be obtained by additionally forming the wall 32 and/or 33 in a curved or serpentine configuration. Unlike other Rake-type combustors the preferred embodiment of this combustor arrangement described is capable of operation over a wide range of operating frequencies and firing rates, particularly under differing conditions of density of the fuel/oxidant mixture. For a given combustor volume, the described chamber separation walls and shape enables increased firing rates (i.e. thermal input) compared with the prior art. It has been observed that, with the geometrically divided combustor 12 and tube assembly 14, once operational the outer combustion chamber 16 and inner combustion chamber 18 tend to seek a common operating frequency and are out of phase so that acoustic noise generated is at least to some extent cancelled through destructive source pressure addition. It is not necessary in this case to take special steps to particularly tune the combustor arrangement to any resonant frequency, since, as mentioned, the combustors automatically tend to settle at a common operating frequency while operating in and out of phase as described above. The common operating frequency can be adjusted by the sleeves 56 connected to the inlet 66 of the combustor 12 and the adjustable sleeves 36 of the tube assembly 14. The use of the frequency adjustment sleeves 56 and 36, if provided as abutting tubes may give more precise control over the acoustic output of combustion arrangement 10.

In the preferred embodiment shown in FIGS. 4 and 5, the wall 20 separating the two combustion chambers 16, 18, exhibit a continuously changing serpentine-like curvature over its length, with the centre of curvature alternating between the outer and inner combustion chambers 16, 18. Inwardly and outwardly curving portions of the wall 20 have differing curvature. The wall 32 defining the outside of the primary combustor may be curved or circular or have straight portions. Similarly the central wall 33 may be curved or circular or have straight portions.

In FIG. 6 mesh plate 63 may be attached to mesh plate 64 and there is also shown adjustment sleeves 36 which are all welded to support plate 57. There is also provided spiral tubes 30 which may be located in the internal chambers of tubes 22 as shown in FIG. 7. Each tube 22 has an outlet part or end 23 engageable with an associated adjustment sleeve 36 by fasteners 25. There is also provided support plate 21 for supporting exhaust decoupling assembly 34. It is also noted that inlets 38 are each alternatively in fluid communication with chambers 16 or 18. Each tube 22 is also attached to a mounting plate 57A which has attachment apertures 61 for engagement with peripheral plate or ring 32 as well as peripheral plate or ring 29.

In FIG. 7 there is shown a longitudinal sectional view of combustion arrangement 10 and it will be noted that each tube 22 has located therein a spiral tube 30 shown in FIG. 7. Cold water is introduced into tubes 62A adjacent outlet end 54 which then travels through branched portion 63 before entering spiral tubes 30. The cold water is heated by the combustion gases within each tube 22 having an internal chamber 22A in heat exchange relationship with spiral tubes 30. Then the water may exit tubes 30 through conduits 63A and 63B and the heated water exits through outlets 17. FIG. 7 also shows a fixed sleeve 69A screw-threadedly engagable with outer sleeve 69 which may be adjusted at adjustment member 70 to move each adjustment sleeve 36 closer to inlet end 50 or further away from inlet end 50 as may be required. Tube 69 has end projection 15. There is also provided a central cylindrical body 71 for supporting each tube 62A and 63B and fixed sleeve 69A. There is also provided bracing members 72. There is also shown fasteners 19 for observation tubes 62. There is also shown inner space 74 surrounded by inlet conduits 52A for fuel/oxidant mixture. Inner space 74 accommodates central plate 28.

During operation the mixture of combustible gas will pass through the mixing chamber 52 into the fuel and oxidant plenum 40 which is in communication with combustor 12 via the fuel porous membrane 28. During operation a charge of fuel and oxidant mixture passes through the fuel porous membrane 28 into the combustor 12 to be combusted. During the initial start up there is provided an ignition device 53A such as a sparkplug or pilot burners known to a person skilled in the art in each of the combustion chambers 16, 18 as shown in FIG. 6. The combustion of the fuel/oxidant mix causes the mixture to burn and the hot gases causes a sudden increase in volume and pressure in the primary combustor 12, as the hot gases expand in the direction of the tube assembly 14. The pressure wave caused by the combustion causes the combusted fuel mix to travel into the tubes 22 with significant momentum and thereby creating a low pressure zone in the combustor 12. This low pressure zone causes the partial return of the combusted fuel/oxidant mixture from the tube assembly 14 into the combustor 12 and a new charge of fuel/oxidant mixture from the plenum 40 into the combustor 12. The incoming and the returning gases collide and mix and the returning hot gases are above the auto ignition temperature of the incoming gas/oxidant mixture. The auto ignition of the gas mixture will then take place and the system starts to oscillate and the cycle repeats itself.

After ignition of the first oxidant/gas mixture with the aid of the sparkplug or ignition device 53A shown in FIG. 6, no further spark is required and the system starts to oscillate and drive itself at a resonate frequency and is self sustaining. The combustor 12 as described in this specification regulates its own stoichiometric range for optimum combustion. The tube assembly 14 may communicate with an exhaust decoupling chamber 34. The tubes 22 of the tube assembly 14 are positioned in alternative arrangement to the outer and inner combustion chambers 16 and 18. The tubes 22 are alternately open to the outer and inner combustion chambers 16, 18 in an arrangement wherein if one of the tubes is open to the outer combustion chamber 16 then the two tubes adjacent it are open to the inner combustion chamber 18. The frequency adjusting sleeves 36 and 56 are arranged so that the combustor 12 and tube assembly 14 are in direct communication with each other. If tuned out of phase the acoustic waves generated in the combustor 12 and tube assembly 14 propagate towards and into the exhaust decoupling device 34 and tend to cancel each other in operation within the exhaust decoupling device 34 and thereby diminish the sound production and emission of the combustion arrangement 10.

In FIG. 8 there is provided combustion arrangement 10A having body or casing 80, heating coils 81 and 82, wall structures 83 and 84 for supporting coils 81 and 82, and intermediate wall structure 85 also for supporting coils 81 and 82 wherein each of structures 83, 84 and 85 each have hollow interiors defining water jackets 86, 87 and 88. There is also provided a length adjustment member 89 which determines the frequency of the heat pulses that are propagated in combustion arrangement 10A. There is also provided fuel plenum 90 which is surrounded by support frame 91. There is also shown exhaust opening 92, cold water inlets 93 and 94 and hot water outlets 95 and 96. There is also provided porous membranes 97, combustion zones 98 which each have an ignition device (not shown), and hot water return conduits 99.

In operation heat pulses are caused to travel through the interior of passageways 100 and 101 which surround heating coils 81 and 82 and return pulses are detonated by porous membrane 97 and then pushed upwardly through passageways 100 and 101 causing a cyclic series of pulses which heat up water contained in coils 81 and 82 for ultimate exit of combustion arrangement 10A through outlets 95 and 96. If desired hot water from water jackets 86, 87 and 88 may also be preheated by coils 81 and 82 before being passed to inlets 93 and 94 by suitable conduits (not shown).

In FIGS. 9-11 there is shown another combustion arrangement 10B of the invention which has a body or housing 105, exhaust gas opening 106, support frame or base 107, water inlet 108 and water outlet 109. There is also shown a releasable cover 110 releasably attached to body 105 by fasteners 111 and a releasable side component 112 attached to body 105 by fasteners 113. There is also provided fuel plenum 114, preheater fuel cavity 115 and inlet 116 for entry of fuel/oxidant into passageway 117. In combustion arrangement 10B fuel/oxidant mixture enters body 105 through inlet 116 and passes through passageway 117 and down fuel cavity 115 to fuel plenum 114 as shown by the arrows in bold. There is also provided water jacket 118 and feed pipes 119 for water jacket 116. Body 105 also has a pair of hollow interiors or chambers 120 shown in FIG. 11 which is occupied by spiral tubes or heating coils 121. There is also provided combustion zones 122 which each has an ignition device (not shown) as well as porous membrane 123. Also shown is horizontal water return conduit 124 which communicates with inlet 108 and horizontal conduit 125 which communicates with outlet 109. The chambers 120 form the conveying zones for conveying hot gases away from the combustion zone 122.

In FIG. 11 there are provided stiffeners 127 for spiral tubes or heating coils 121 and porous membrane 123 which extends the width of chamber 120. There is also provided hot water return conduits 128 from the bottom of heating coils 121. There are provided a pair of arrays of heating coils 121 each located by a peripheral wall 129 which defines each chamber 120.

In each of the embodiments of FIG. 8 and FIGS. 9-11 it will be appreciated that porous membranes 97 and 123 are located in an inlet to combustion zones 98 and 122. In FIG. 8 the inlet or inlet opening is shown as 131 in each zone 98 and 132 in each zone 122.

A further advantage of the combustion arrangement 10, 10A and 10B resides in the effectiveness of the combustion method of the invention and combustor 12, 98 and 122 in particular to the ability to maintain flames at high combustion efficiencies with low NOx and pollutant emissions. All aspects are closely related to the flow characteristics inside the combustion chamber and the degree of mixing obtained with the combustible gas and the oxidants. The mixing involves several important processes. Large-scale structures bring into the mixing layer large amounts of reacting components from the separate reactants. The fine-scale eddies enhance the mixing at a molecular level between the reactants which is a necessary condition to initiate the complex sequence of exothermic chemical reactions between a fuel and an oxidant.

When the flame propagates in a non-uniform flow it experiences strain and curvature effects. The fractional rate of change of the flame area constitutes the flame stretch. The flame stretch parameter determines the available flame surface density and consequently the reactivity of, the fuel and oxidants.

Stretching of the flame and the body of the flame also controls some of the mixing and consequently the emission of pollutants. Large flame stretching parameters generally lead to lower temperatures and modified residence times of the reactants in the burning zone and hence reduced emissions of thermally generated NOx. The modification of the flame stretch can be achieved by modification of the fluid dynamics.

Change in the flame stretch of the flame is facilitated by the geometric division of the primary combustor chamber 12 in accordance with the invention, the subsequent modified flow field that can lead to a change in the chemistry occurring by altering the chemical equilibrium.

The understanding of these complex gas dynamic processes and their interactions requires analysis of the interactions between the fluid dynamics, chemical reactions, acoustic waves, flame stretch and heat release of the reactive system.

The combustors and combustion chambers and method in accordance with this invention facilitate very high rates of flame stretch, while at the same time sweeping up the remnants of the flamelets and returning them to be incorporated into the next flamelets. This also causes a micro form of exhaust gas recirculation, which is known to a person in the art, to dramatically reduce or prevent unwanted emission, particularly NOx.

The continual recycling of flamelets allows much longer residence times for reactions to take place. Very high flame stretch also reduces radicals in the flame. These radicals are parts of molecules, e.g. —CH and —OH, that can exist at the higher temperatures encountered in flames. Most of these radicals are responsible for much of the pollutant formation.

The situation is further improved by the very high degree of mixing that is generated. The pollutant forming reactions tend to be greater than 1st order, which get slowed up by increased mixing, where as the desirable reaction, which tend to be less than 1st order, get speeded up. The combination of these effects leads to the very clean low pollution Combustion.

The geometry of the combustor 12 facilitates the emission of oxide of nitrogen NOx and carbon monoxide CO to be eliminated or reduced to levels less than 1 ppm from the combustion processes, by introduction of the geometrical division 20 between the outer and inner combustion chambers 16, 18, thereby facilitating the combustion of combustible gases in such a manner that oscillation and flame stretching occurs within the porous membrane 28. The partition 20 of the primary combustor 12 causes the primary combustor 12 to oscillate a different frequency from the secondary combustor 14.

For optimum functioning the correct aspect ratio is chosen to match the fundamental harmonic frequencies generated by the length and geometry of the Rijke type combustor arrangement. This is achieved by ensuring that the major length of the combustors (L) is a whole number multiple of both the inlet orifice depth (α), the flame retainers thickness (t) and the fundamental harmonic oscillation frequency (f,) whereas the inlet orifice depth is α={(½L*n/f)−(50*t)}/100 where n=the number of nodes. The major length of the combustors L is the distance from the inlet lip 48 to the top of the adjusting sleeves 36.

In one embodiment for the number of nodes n=320, the orifice depth a is 3.0184 millimetres where major length ½L=640 millimetres, flame retainer thickness t=2.2 millimetres and frequency f=497.26. Thus for the formula α={(½L*n/f)−(50*t)}/100:

0.0030184m={(0.64*320/497.26)−(50*0.0022)}/100

The fundamental frequency f=(Speed of sound)/(wave length (wave length in this instance 1.28 meter). ½ L=halve a wavelength=0.64 meter. The speed of sound in air at 500° C. is approximately 636.5 metres/second. For the embodiment above the frequency f was thus calculated as 636.5/1.28=497.265625 Hz.

Another example tube length 800 mm=wavelength is 1.6 meter, frequency of this tube length is speed of sound at the given temperature is 636.5 m/sec divided by wavelength 1.6 meter=397.81

{(0.8*320/397.81) (50*0.0022)}/100=5.33 mm lip length another example is 100 cm or 1 meter Rijke tube length, the speed of sound 636.5 m/sec divided by wave length (2 mtr)=318.25

{(1*320/318.25)−(50*0.0022)}/100=8.954 mm length of lip

By these constructions it has been found that stable oscillating combustion can be achieved over a wide range of operating conditions, with there being a band or bands of oscillating frequencies at such operations can be achieved. With this particular geometric construction in accordance with the invention it is possible to obtain a higher combustion frequency with increased heat transfer, higher than conventional designed equal volume combustors.

This invention is directed towards the achievement of pulsing or rhythmic combustion that causes oscillation of gases and fuel flow, which subsequently causes the flame to stretch, within the boundary limit of the porous membrane or flame retainer 28, and the prompt NOx to be suppressed, which result in the reduction of the prompt NOx radical reactants, and the exhaust gas recirculation within the flame zone, which biases the prompt NOx reactions to go in reverse directions, thus stopping any further NOx from being produced. This NOx is trapped in the flame within the porous membrane 28, and does not exit. The NOx levels within the flame area can be quite high even though the net emission is zero. This is quite different from conventional methods attempting to reduce NOx by means of exhaust recirculation, where the burnt fuel or exhaust are taken from outside the flame area and put back into the fuel and/or air steam prior to the combustion zone. The oscillating combustion enhances the heat transfer and break-up of the thermal boundary layers in the heat exchange equipment.

Throughout this specification, unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element or integer or group of elements or integers but not the exclusion of any other element or integer or group of elements or integers.

The use of the present combustor arrangements for the production of steam or hot fluid directly addresses the looming problem of greenhouse gas emissions and energy conservation. The withdrawal of methyl bromide with subsequent lack of suitable and economic control means for soil sterilisation in horticulture and agriculture. The devices may, of course, also be used for generating of heat for the purpose of hot water, steam, organic fluids and gasses or conditioning soil and seed for storage to avoid insect damage, fungal growths and other pathogens, and enhancing the safety of supplies in the food chain. 

The following amended claims replace all claims in the application:
 1. A process of producing heat energy for use in heat exchange with other fluids and substances so as to impart said heat energy to said fluids or substances which includes the steps of: (i) igniting a mixture of fuel and oxidant in a combustion zone or zones caused by an ignition device located in said combustion zone or zones to create a combusted fuel mix which in the form of shockwaves are conveyed away from the combustion zone or zones to provide a low pressure area within the combustion zone or zones; and (ii) said low pressure area causing partial return of the combusted fuel/oxidant mixture from a location remote of the combustion zone or zones and also causing a new charge of oxidant/fuel mixture to be transferred into the combustion zone or zones whereby incoming and returning hot gases are caused to ignite automatically without the aid of the ignition device used in step (i) thereby causing a series of pulses or oscillations of successive return of hot gases and further charges of fuel/oxidant mixture to provide a process of production of energy which is self-sustaining and continuous.
 2. A process as claimed in claim 1 wherein the combustion zone is separated into an inner and outer combustion zone wherein ignition devices are located in each of the inner and outer combustion zones.
 3. A process as claimed in claim 1 wherein there are provided a multiplicity of separate combustion zones which are in communication with each other whereby only a single ignition device is required.
 4. A process as claimed in claim 1 wherein the automatic ignition occurs at a detonation zone formed by a porous membrane located in inlet(s) of a planar member having spaces which form part of the combustion chamber.
 5. A Rijke type combustion arrangement having at least one combustion zone having an associated ignition device wherein the or each combustion zone has one or more inlets or entrances and a flame retainer in the form of a porous membrane located in or adjacent to the or each inlet or entrance and there is further provided one or more conveying zones located adjacent the porous membrane for conveying hot gases away from the combustion chamber or space.
 6. A combustion arrangement as claimed in claim 5 which is provided with inner and outer combustion zones each having an associated ignition device and a tube assembly having a plurality of tubes forming a multiplicity of separate conveying zones or passageways.
 7. A combustion arrangement as claimed in claim 6 wherein each tube has an inner end wherein the inner ends of each of the tubes are each open to a different one of the inner and outer combustion zones and each porous membrane is in fluid communication with an associated inner end of each of the tubes.
 8. A combustion arrangement as claimed in claim 5 which only has one conveying zone which is a single chamber located above the porous membrane and a combustion zone located closely adjacent to the porous membrane which includes the ignition device.
 9. A combustion arrangement as claimed in claim 6 wherein each of the tubes are in flow communication with each other at an outlet end.
 10. A combustion arrangement as claimed in claim 6 wherein there is provided a planar member adjacent each of the inner and outer combustion zones having apertures therein which form the inlet(s) of the inner and outer combustion chambers.
 11. A combustion arrangement as claimed in claim 10 wherein the apertures are each defined by a cylindrical wall and the flame retainers are located between opposed ends of the cylindrical walls.
 12. A combustion arrangement as claimed in claim 6 which includes a frequency adjustment device located at an outlet end of each tube as well as being located adjacent respective inner and outer combustion zones.
 13. A combustion arrangement as claimed in claim 12 wherein each frequency and adjustment device located at an outlet end of each tube are in the form of a plurality of sleeves which are movably or slidably mounted to a respective tube.
 14. A combustion arrangement as claimed in claim 12 wherein each inlet or entrance has a frequency adjusting sleeve which is movable relative to an adjacent inlet to allow adjustment of an inlet orifice depth which is a distance between a lip of the frequency adjustment sleeve and an associated porous membrane.
 15. A combustion arrangement as claimed in claim 6 wherein the inner and outer combustion zones are separated by continuous partition having a peripheral edge which is serpentine.
 16. A combustion arrangement as claimed in claim 6 wherein there is provided a ring of tube internal chambers which are alternatively open to the outer combustion zone and the inner combustion zone.
 17. A combustion arrangement as claimed in claim 6 wherein there is provided a plurality of tube internal chambers which form an inner set of tube internal chambers and an outer set of tube internal chambers which are concentric with the inner set of chambers wherein the inner ends of the outer set of chambers are open to the outer combustion zone and the inner ends of the inner set of tube internal chambers are open to the inner combustion chamber.
 18. A combustion arrangement as claimed in claim 5 which includes a fuel plenum and the inlet(s) of the combustion chamber(s) are open to the fuel plenum.
 19. A combustion arrangement as claimed in claim 18 which includes a foci mixing chamber which is open to the fuel plenum for feeding fuel to the fuel plenum. 